How do I maintain timer effectiveness?

I'm struggling to keep my countdown timers effective without seeming manipulative or losing customer trust. Every time I implement a timer, I worry about appearing too aggressive or creating skepticism among potential buyers. I need strategies that create genuine urgency without feeling like a cheap sales tactic, and I want to understand how to design timers that actually motivate purchases rather than driving customers away. What are the psychological principles and technical approaches to maintaining timer credibility and conversion power?